1. Field of the Invention
The present invention relates to a Fresnel lens sheet to be employed in combination with a lenticular lens sheet in constructing a rear projection sheet for a rear projection television screen, and to a rear projection screen employing the Fresnel lens.
2. Description of the Related Art
Fresnel lens sheets are classified into those of a single-focal type which refract all light rays that have fallen thereon so as to collect at a single focus as shown in FIG. 6A, and those of a multifocal type which refract light rays that have fallen on regions near the periphery at focuses at longer focal lengths, and refract light rays that have fallen on regions near the center, at focuses at shorter focal lengths, respectively as shown in FIG. 6B. The center of a Fresnel lens is an optical center of the Fresnel lens where the lens angle at the lens top relative to a plane parallel to the major plane of the Fresnel lens, is nearly zero.
When a rear projection screen employing the conventional Fresnel lens sheet is employed in a rear projection television receiver, most part of light rays collected at an optimum observation point of the rear projection television receiver are those traveling through the central region of the rear projection screen and, consequently, an observer feels that a region around the center of the rear projection screen is brightest. Such a phenomenon is called "hot band phenomenon" which makes the central region of the rear projection screen look very bright and makes the peripheral region of the same look dark, spoiling the uniformity in brightness of the rear projection screen.